Re: [boost] [outcome v2] Please comment on new result<T, EC> reference API docs

2017-06-22 4:23 GMT+02:00 Niall Douglas via Boost :

On 22/06/2017 02:08, Peter Dimov via Boost wrote:

...

Niall Douglas wrote:

...

The idea, since we no longer have variant storage, is to see if we'd like to implement

http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2016/p0262r0.html (except with the parameter order reversed).

I'm not particularly enthusiastic about status_value (and the argument given in the paper is unconvincing.) Neither am I thrilled by the idea of result (an optional) or result (something like a glorified pair.)

I've fixed the problems you raised at https://dedi4.nedprod.com/static/result-v2/doc_result.html and moved the implementation stuff into a impl namespace so standardese will still show the docs for the composited types, but not be usable by end users.

...

result in its original form represented a very clear concept: the type of a function that must either return a value, or a reason for the lack of value. All of the above do not fit this concept and to me ring hollow.

Andrzej feels similarly to you.

But I don't think things are as bad as both of you feel. In v2 result you now can have:

1. Successful: T

2. Failure: EC

3. Successful + Status: T + EC

The reason I don't think this is a problem is because you can't accidentally return a wrong result in practice. Either you're using result to return Success + Status, or you're using result to return Success | Failure. It is very hard to accidentally write code which does the wrong thing here, almost certainly the type system would refuse to compile incorrect code.

You could say "if you do not like the state `has both value and error`, then don't create it", and this could be considered acceptable, but now that you have said you intend to change "error" into "status", you are affecting even this conservative usage. In the paper by Laurence Crowl, in the example with queue status: ``` Value queue::value_pop(); queue_op_status queue::wait_pop(Value&); ``` what he is describing is: pair Whereas what you provide (if I got it correctly) is: pair Which means I can get a value with no status. This still does not implement P0262. Regards, &rzej;